This invention relates to pellet mills, in particular, the shaft and bearing support for roller pelletizers.
In a conventional pellet mill, such as described in U.S. Pat. No. 3,981,664, "Main Shaft Support For Pellet Mills", a main shaft has one end which supports a roller frame. The roller frame rotatably holds a plurality of rollers. Disposed around the outside of the roller frame and rollers is an annular die holder having a replaceable die. The die is supported in such a manner as to be in intimate contact with the rollers on the roller frame. The die holder surrounds the shaft and is supported by bearings for rotation relative to the shaft. Coaxially disposed around the shaft and the die holder is a main frame structure, or casing. The die holder is allowed to rotate relative to this frame structure by the use of a second set of bearings. The end of the main shaft not supporting the roller frame extends beyond the die holder and is fixed relative to the main frame. Therefore, the die holder rotates relative to the shaft and the main frame, but the shaft and main frame are fixed relative to each other.
The die and die holder define a pelletizing chamber within which the rollers and roller frame are stationed. Viscous solid feed material, such as animal feed products, previously conditioned by an extruder, are introduced into this pelletizing chamber. The pelletizer operates by employing a motor to rotate the die holder and therefore the die. The rollers ride along the inside surface of the die and rotate due to the rotation of the die itself, but have no independent means of rotation. Feed material is trapped between the rotating rollers and the rotating die and extruded through radially disposed slots in the die. Once the feed material has been forced through the slots in the die, knives or other means outside of the die shear the feed material from the die to form pellets. The cut pellets are then deposited in and collected from a housing surrounding the mill.
Conventional pellet mills require at least two sets of roller bearings to allow relative rotation between the die holder and the frame, and between the die holder and the shaft. Accommodating two sets of bearings, each with its own tolerance limits, gives rise to noticeable vibration. These bearings are also subject to high stresses and require lubrication while located within the complicated structures of the mill. The main shaft must also be of sufficient robust construction so as to be able to resist bending, torsion and shear forces as a result of the rotation of the die on the outside of the rollers. Additionally, the main shaft must support the weight of all of the components. The result of all these forces is decreased bearing life, and stresses that can exceed the limits of the bearings leading to their failure.